fix(code/frontend): side navigation bar width should be fixed (#31876)

Author: WangQianliang

x-pack/plugins/code/public/components/main/main.scss

@@ -29,6 +29,8 @@
   width: calc(256rem / 14);
   border-right: $euiBorderThin;
   display: flex;
+  flex-shrink: 0;
+  flex-grow: 0;
   flex-direction: column;
   & > div {
     height: 100%;